The invention relates to a method of determining a nuclear magnetization distribution from magnetic resonance signals which are generated in a body which is situated in a steady, uniform magnetic field, which magnetic resonance signals are generated in sub-regions of the body by means of selective pulse sequences, in a pulse sequence for generating resonance signals in the sub-region there being excited nuclear spins by application of a selective RF electromagnetic excitation pulse, after which at least one magnetic field gradient which is superposed on the uniform magnetic field is applied, at least one such gradient being variable in amplitude or direction from one pulse sequence to another, there being applied an RF electromagnetic echo pulse in order to generate a resonance signal from the excited nuclear spins, after which the pulse sequences are repeated a number of times for different values of the variable magnetic field gradients and subsequently the nuclear magnetization distribution is determined from the resonance signals generated.
The invention also relates to a device for determining a nuclear magnetization distribution from magnetic resonance signals to be generated in a body, which device comprises means for generating a steady, uniform magnetic field, means for generating selective RF electromagnetic pulses, means for generating at least one magnetic field gradient whose amplitude or direction are variable, and control means for controlling the means for generating the selective RF electromagnetic pulses, means for receiving, detecting and sampling the magnetic resonance signals, and also comprises processing means which include programmed arithmetic means for determining the nuclear magnetization distribution from the sampled resonance signals.
A method and device of this kind are known from U.S. Pat. No. 4,665,367. According to such a method and device, a body to be examined is arranged in a steady, uniform magnetic field B.sub.0 whose direction coincides with the z-axis of a stationary cartesian coordinate system (x, y, z). Under the influence of the magnetic field, a small excess of the nuclear spins present in the body are directed in the same way with respect to the theoretically possible saturation value (all nuclear spins) due to thermal movement. From a macroscopic point of view, the small excess is to be considered as a magnetization M of the body or as a slight polarization of the nuclear spins. After the body arranged in the magnetic field has been irradiated by an RF electromagnetic pulse which must have a given frequency, the equilibrium of the magnetization M is disturbed so that it starts to perform a precessional motion about the magnetic field B.sub.0. When the processional motion is observed from a cartesian coordinate system (x', y', z') which rotates in the same direction and whose z'-axis coincides with the z-axis of said stationary cartesian coordinate system and when the angular velocity of the cartesian coordinate system rotating in the same direction is chosen to be equal to the angular frequency .omega. of the RF electromagnetic pulse, the magnetization M is to be considered to be a vector when the angular frequency .omega. of the RF electromagnetic pulse equals the resonance frequency .omega..sub.0 of the nuclear spins, which vector moves under the influence of the irradiation in a plane perpendicular to the direction of irradiation. The component of the magnetization M perpendicular to the z-axis, the so-called transverse magnetization, causes a resonance signal after irradiation. For the resonance frequency .omega..sub.0, the so-called Larmor equation .omega..sub.0 =gamma.B.sub.0, holds good, where gamma is the gyromagnetic ratio of, for example, protons. The angle of rotation of the magnetization M, and hence the magnitude of the resonance signal, is determined by the area underneath the RF electromagnetic pulse. An RF electromagnetic pulse which rotates the magnetization M through 90.degree. in the stationary coordinate system will be referred to hereinafter as a 90.degree. pulse. After irradiation, the magnetization M will relax with a time constant T.sub.1, the so-called longitudinal relaxation time, until is reaches the static of equilibrium. A further time constant is the so-called transverse relaxation time T.sub.2, which is the time constant indicating the decay of the transverse magnetization. In practical cases the transverse magnetization decays with a time constant T.sub.2 * which is substantially smaller than T.sub.2 due to dephasing under the influence of inevitably present field inhomogeneities. However, within the relaxation with T.sub.2 always resonance signals can be obtained by rephasing. By application of magnetic field gradients G.sub.x, G.sub.y and G.sub.z on the magnetic field B.sub.0, the field directions of said gradients corresponding to that of the magnetic field B.sub.0 and their gradient directions extending perpendicularly to one another, a location-dependent magnetic field B=B.sub.0 +G.sub.x.x+G.sub.y.y+G.sub.z.z can be generated. U.S. Pat. No. 4,665,367 describes how resonance signals can be generated in sub-regions of the body by means of selective pulse sequences. Selective pulse sequences are pulse sequences in which excitation pulses occur which excite only the nuclear spins of a sub-region in the presence of a gradient and which do not excite the nuclear spins of other sub-regions. The excitation pulses then cover a range of Larmor frequencies associated with a local field. The gradient which provides selection, together with the RF electromagnetic pulses, is also referred to as the selection gradient (for example G.sub.z). FIGS. 3 and 10 of said U.S. Pat. No. 4,665,367 show selective excitation for sub-regions (multiple-slice) and appropriate pulse sequences, respectively. For 16 sub-regions resonance signals are collected while varying the amplitude of one or two magnetic field gradients (for 2D and 3D imaging, respectively). In the present example, 4 resonance signals can be generated in each waiting period required for relaxation of the magnetization to the state of equilibrium. In that case, 4 waiting periods are required for generating a resonance signal for all 16 sub-regions. By repeating the sequence a number of times (for example, 256 times) while varying Gy, a sufficient number of resonance signals can be collected for determining, for example for each sub-region, a nuclear magnetization distribution from sampling values of the respective resonance signals by means of, for example 2D Fourier transformation (Fourier zeugmatography). By repeating the sequence also while varying G.sub.z, 3D Fourier transformation can be applied in order to determine a 3D nuclear magnetization distribution of the body. It will usually be desirable to form nuclear magnetization distributions of a number of sub-regions (for example, slices) of the body, and also images thereof in which a T.sub.1 weighting operation (T.sub.1 contrast) and a T.sub.2 weighting operation (T.sub.2 contrast) are performed. For example, different echo times must then be used for T2 weighting and different pulse sequence repetition times and/or inversion pulses must be used for T.sub.1 weighting. The echo time is the period of time expiring between the generating of the excitation pulse and the occurrence of an echo resonance signal in a sequence. In the case of a comparatively long echo time, for example as in the case of T.sub.2 weighting, a loss of time is incurred. T.sub.1 weighting and T.sub.2 weighting together offer a suitable discrimination of tissue in, for example, in vivo measurements.